A switching mode power supply and method for generating a bias voltage is
described. The bias voltage is generated by using a current source
coupled to a primary coil of a transformer during the start-up of the
switching mode power supply. The generation of the bias voltage through
the current source is stopped at a second time, a delay time after a
first time when the bias voltage becomes essentially equal to a reference
voltage that is an operational voltage of a PWM controller. The switching
operation of a main switch is started after the first time, when the bias
voltage exceeded the operational voltage. In this interval the bias
voltage is also generated by the secondary coil of the transformer.